Exhaust gas oxygen sensor

ABSTRACT

The present invention identifies a formulation for a subgroup of perovskite structure oxides that overcomes the outstanding problems for oxygen sensing in a combustion environment. The sub group has a formula ABO x  where A is a large 3-valent ion, such as Pr 3+ , B is a transition metal ion, which is substituted to a small degree by tungsten (which has a stable valence of 6), and x indicates that the oxide can sustain a variable oxygen stoichiometry. A preferred general formulation is a single-phase perovskite structure AB 1-y W y O x  where y preferably lies between 0.03 and 0.15, more preferably between 0.05 and 0.10 and where x is close to 3. Preferred examples of compositions that can achieve these advantages include, but are not limited to, PrFe 0.95 W 0.05 O x  and LaFe 0.95 W 0.05 O x .

BACKGROUND OF THE INVENTION

Semiconductor gas sensors function by offering a change in electricalresistance in response to a shift in the local concentration of the gasof interest. In general the resistance of the sensor is some function ofthe concentration of the target gas.

Oxides with the perovskite type crystal structure and general formulaABO₃, where A represents a large main group ion, and B represents atransition metal, are particularly suitable for use as oxygen sensors.This is because the perovskite structure is very robust and allows aconsiderable variability in oxygen content without breakdown instructure. Early examples of compositions proposed for the applicationwere SrTiO₃ and BaTiO₃.

The problem of temperature sensitivity with perovskite structures wasovercome by the selection of a group of perovskite structurecompositions that exhibited constant values of conductivity (in anatmosphere of constant oxygen partial pressure) over a considerablerange of temperature. U.S. Pat. Nos. 4,454,494 and 4,677,414 describethis desirable property for a group of alkaline earth ferrates (forexample, Sr Feo_(3-x) and Ba FeO_(3-x)) in which the iron component waspartially substituted by other transition metals whose primary valencestates were 4 or 5.

Subsequently U.S. Pat. No. 5,397,541 laid claim to a very wide range ofperovskite structure oxides as potential sensors for oxygen. At leastone of the examples in U.S. Pat. No. 5,397,541 (SrFe_(0.9)Ti_(0.1)O_(y))lies within the range of compositions covered by U.S. Pat. No.4,454,494.

Needs exist for improved methods and apparatus for sensing that areselective for a particular gas, without interference from othercomponents of the atmosphere, including moisture (relative humidity),and/or changes in temperature.

SUMMARY OF THE INVENTION

In general the resistance of the sensor is some function of theconcentration of the target gas. Two general mechanisms can be exploitedto achieve this concentration dependence on gas composition. A presentinvention engineers a response, which is selective for a particular gas,without interference from other components of the atmosphere, includingmoisture (relative humidity), and/or changes in temperature.

The mechanism of semiconductor gas sensors operating at lowertemperatures, generally within the range 200-500° C., involves reactionsof molecules of the target gas with chemisorbed species on the surfaceof the semiconductor, which is usually a metal oxide, and results in achange in near-surface charge carrier density. Materials functioningthrough this mechanism can be employed in the detection and monitoringof either reducing gases, for example but not limited to hydrogen,carbon monoxide, methane, etc., or oxidizing gases, for example but notlimited to nitric oxide, chlorine, ozone, etc., in ambient air. Thisresponse mechanism involves no change in the bulk composition of thesemiconducting oxide.

Within a somewhat higher temperature range, approximately 500-700° C., afamily of semiconducting oxides MO_(x), where M represents either atransition metal or a combination of metals, one of which is atransition metal, can be used for monitoring oxygen partial pressure. Inthis case the bulk stoichiometry does change because the oxygen contentof the material (the value of x) equilibrates with the prevailing oxygenpartial pressure. Changes in the value of x are compensated by changesin the ratio of the valence states of the transition metal component ofM. The present sensor preferably operates via this second mechanism.

These and further and other objects and features of the invention areapparent in the disclosure, which includes the above and ongoing writtenspecification, with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a schematic of a variation of conductivity ofperovskite structure oxides with temperature and oxygen partialpressure.

FIG. 2 is a graph showing resistance versus time for a sensor at 600° C.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In general, the conductivity of materials, such as SrTiO₃ and BaTiO₃,follows the form shown schematically in FIG. 1. Through the region oflow partial pressure the conductivity falls as the oxygen partialpressure increases. It passes through a minimum value and then increasesas the partial pressure continues to rise. The material is thusexhibiting n-type behavior at low oxygen partial pressure and p-typebehavior over the higher oxygen partial pressure range. Both the oxygenpartial pressure and the temperature generally influence the conductanceof these oxides. This is shown by reference to the series of curves oneabove the other in FIG. 1. These correspond to increasing temperaturesmoving up the FIG. 1.

A result of the minimum value present in each curve is that mostmeasurements of conductance of the oxygen sensor are capable of twodifferent interpretations because there are two values of oxygen partialpressure that could give rise to the same conductance value. Suchambiguity is likely to reduce the usefulness of the sensor unless it canbe avoided.

An important feature of the dependence of conductivity on oxygen partialpressure, seen in FIG. 1, is that in the p-type range to the right ofFIG. 1, the isothermal data lines are very much closer together than inthe n-type range on the left of FIG. 1. In other words the temperaturedependence of conduction is considerably less-in the p-type than in then-type range. Operation in the p-type range facilitates the task ofeliminating temperature effects from the oxygen partial pressuremeasurements.

One of the major applications foreseen for oxygen sensors is in themonitoring and control of combustion processes and it transpires that,unfortunately, when the fuel in use contains even a small amount ofsulfur, so that the atmosphere in which the sensor operates includessulfur oxide gases, perovskite oxides structured around alkaline earthions, for example Sr and Ba, undergo a permanent degradation due to theformation of stable sulfates, for example SrSO₄, BaSO₄, and quicklyfail.

The present invention identifies a formulation for a subgroup ofperovskite structure oxides that overcomes the outstanding problems foroxygen sensing in a combustion environment. The sub group has a formulaABO_(x) where A is a large 3-valent ion, such as Pr³⁺, B is a transitionmetal ion, which is substituted to a small degree by tungsten, which hasa stable valence of 6, and x indicates that the oxide can sustain avariable oxygen stoichiometry.

A preferred general formulation is a single-phase perovskite structureAB_(1-y)W_(y)O_(x) where y preferably lies between 0.03 and 0.15, morepreferably between 0.05 and 0.10 and where x is close to 3.

Preferred examples of compositions that can achieve these advantagesinclude, but are not limited to, PrFe_(0.95)W_(0.05)O_(x) andLaFe_(0.95)W_(0.05)O_(x).

A first advantage of this invention is that, lacking an alkaline earth,the compositions are not prone to the formation of sulfates as stable asSrSO₄ and BaSO₄, and therefore can be used in atmospheres where there issome contamination by sulfur gases.

A second advantage is that, with a minimum amount of doping on theB-site, the minimum in the conductance-oxygen partial pressure plot canbe driven far in the low oxygen partial pressure direction, as seen tothe left in FIG. 1, allowing the p-type range of the oxide to be usedover the whole range of oxygen partial pressure of interest incombustion control. This is achieved because the replacement of iron onthe B-site by an ion of higher valence invests the oxide with a morepredominantly p-type characteristic down to lower oxygen partialpressures.

The use of a 6-valent ion, with nominally 3 excess positive charges overthe ferric ion per substitution, is more effective than the use of4-valent or 5-valent ions that have 1 or 2 excess positive chargesrespectively. Thus the required shift to p-type characteristic can beachieved with a lower level of doping on the B-site. The result is thatthe ambiguity in the interpretation of conductance measurements iseliminated for the range of partial pressure of interest.

EXAMPLE

Stoichiometric amounts of constituent oxides La₂O₃, Fe₂O₃ and WO₃sufficient to prepare approximately 20 grams ofLa_(1.0)Fe_(0.95)W_(0.05)O₃ were mixed thoroughly with 100 grams NaHCO₃and heated in an alumina crucible to 900° C. for 10 hours. The mixturewas cooled to room temperature and washed with distilled water to removeall traces of sodium compounds. XRD confirmed that the product had aperovskite crystal structure.

The powder was ground and then dispersed in an organic vehicle andscreen-printed over gold interdigitated electrodes on an aluminasubstrate to give an oxide layer thickness of 50 microns. The substrate,which had previously been equipped with a platinum resistance heaterprinted on the reverse, was fired in a belt furnace at 980° C. to give abrown colored sensor.

When exposed to a change in pO₂ by switching the atmosphere from air(20% oxygen) to pure nitrogen (0% oxygen), with the sensor at 600° C. aresistance increase resulted, as expected for a p-type semiconductingoxide, see FIG. 2, which shows duplicate responses.

The sensor, still at 600° C., was then exposed to an atmospherecontaining 100 ppm H₂S for 5 minutes and then returned to air. After 5minutes the resistance had returned to within about 10% of its originalvalue indicating that the device had not been poisoned by exposure tothe sulfur-containing gas.

A preferred method of preparation of the sensor material comprisesreacting starting material oxides in stoichiometric proportions in amolten salt, yielding a powder, screen-printing the powder on asubstrate, forming a microstructure, and forming the sensor.

A preferred method of sensing combustion status (fuel-rich or fuel-lean)of an atmosphere of combustion gases comprises contacting the sensormaterial with the atmosphere, sensing change in conductance, resistance,capacitance and/or impedance in the sensor material, and monitoring andcontrolling combustion processes responsive to the change sensed in thesensor material.

While the invention has been described with reference to specificembodiments, modifications and variations of the invention may beconstructed without departing from the scope of the invention.

1. A gas sensor for monitoring and controlling combustion processescomprising a sensor material of a perovskite structure oxide of formulaABO_(x), wherein A is a large 3-valent ion, wherein B is a transitionmetal ion substituted to a small degree by tungsten, and wherein xdenotes a variable oxygen stoichiometry.
 2. The sensor of claim 1,wherein the perovskite formula is AB_(1-y)W_(y)O_(x).
 3. The sensor ofclaim 2, wherein y is in a range between 0.03 and 0.15.
 4. The sensor ofclaim 3, wherein y is in a range between 0.05 and 0.10.
 5. The sensor ofclaim 2, wherein x is about
 3. 6. The sensor of claim 2, wherein theperovskite structure is PrFe_(0.95)W_(0.05)O_(x).
 7. The sensor of claim2, wherein the perovskite structure is LaFe_(0.95)W_(0.05)O_(x).
 8. Thesensor of claim 1, wherein the perovskite structure does not form stablesulfates in environments contaminated by sulfur.
 9. The sensor of claim1, wherein minimum doping on the B-site provides a required range ofoxygen partial pressure operation.
 10. The sensor of claim 9, furthercomprising a 6-valent ion for doping on the B-site.
 11. The sensor ofclaim 10, wherein the 6-valent ion enables a p-type range of theperovskite structure for use over a range of oxygen partial pressures ofinterest for monitoring and controlling combustion processes.
 12. Amethod of preparation of the sensor material of claim 2, comprisingreacting starting material oxides in stoichiometric proportions in amolten salt, yielding a powder, screen-printing the powder on asubstrate, forming a microstructure, and forming the sensor.
 13. Amethod of sensing combustion status of an atmosphere of combustion gasescomprising contacting the sensor material as described in claim 2 withthe atmosphere, sensing change in conductance, resistance, capacitanceand/or impedance in the sensor material, and monitoring and controllingcombustion processes responsive to the change sensed in the sensormaterial.